The invention relates to a luminescent material having a fundamental lattice consisting of an inorganic crystalline compound, which material comprises at least 1 mol % of gadolinium, at least 0.1 mol % of an activator chosen from the group of transition metals and rare earths, and at least 0.1 mol % of a sensitizer.
A luminescent material of this type is known from Netherlands patent application 186707. In the known luminescent material, the sensitizer is chosen from the group of lead, antimony and bismuth, and the activator is chosen from the group of manganese, terbium and dysprosium. The inorganic crystalline compound and the concentrations of sensitizer and gadolinium are chosen to be such that, if no activator is present in the material but comprises only a sensitizer and gadolinium, the material has the characteristic line emission of gadolinium in the range of 310 nm to 315 nm upon excitation by UV radiation at a wavelength of approximately 254 nm. In other words, upon excitation of the material, an energy transfer from the sensitizer to gadolinium takes place. If, as in the known luminescent material, the material comprises an activator in addition to a sensitizer and gadolinium, an efficient transfer of energy also takes place from gadolinium to the activator, even at a relatively low concentration of the activator. Such a low concentration of the activator renders the luminescent material relatively inexpensive. Moreover, at such a low concentration of the activator, there is little concentration quenching so that a high luminous flux can be obtained. A drawback of the known luminescent material is, however, that the quantum efficiency is limited in that only one visible photon is generated for each exciting UV photon.